Multiple promising crosslinks

Annexin V-functionalized crosslinked iron oxide (CLIO) was designed as a contrast agent for MRI, which was additionally labeled with Cy5.5 to allow colocalization with optical imaging techniques [98]. Alternatively, conjugation of multiple Gd-DTPA molecules or SPIO particles to the C2 domain of synaptotagmin I was shown to allow the detection of apoptotic cells in vitro [99]. Zhao et al. [100] were the first to apply a C2 domain-functionalized SPIO and showed very promising results for future in vivo applications of MR contrast agents for the detection of apoptotic sites. 

The presence of only one Si(OR )3 unit per chromophore makes the formation of a highly crosslinked sol-gel network very difficult. The high mobility of the NLO dye in the free volume of the host network causes relaxation of the poling-induced order. Presently, the most promising approach to improve the stability of the second-order optical nonlinearity is therefore the use of multiple-substituted dyes, such as 2083. 

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